Economical precision manufacture of mechanical parts, particularly those made in small quantities, requires that the relative displacement of the members of the machine upon which they are produced be quickly and accurately determinable at all times. In the past, attempts to meet this requirement have involved use of one or more of the following, associated in most cases with graduated dials, cursors, and other read-out means:
1. Lead screws and nuts PA1 2. Rack and pinion mechanisms. PA1 3. Standard length measures, (such as Johansson blocks). PA1 4. Optical systems, generally associated with very high magnification. PA1 5. Electronic systems in which a series of electrical fields interact upon relative displacement. PA1 6. Friction-dependent systems, in which PA1 7. Rotary drum systems in which a cable or tape is wrapped spirally about a drum, as in an elevator hoist. PA1 8. Systems combining two or more of the above. PA1 1. Absolute error produced by irreducible errors in manufacture. PA1 2. Backlash errors -- as when a screw and rack system is moved to the same position from opposite directions. PA1 3. Accumulative errors -- as when a very slight error developed in movement to a first operating position is increased by a similar error occurring during movement to a second operating position. PA1 4. Errors which increase in proportion to use -- as is the case in all systems dependent solely upon frictional engagement. PA1 5. Errors due to stress, as when varying the load on the member being moved causes a variable error in the means used to determined the ultimate position of the member. PA1 6. Errors caused by misalignment of the relatively moving member and the measuring means. PA1 7. Operator errors attributable to inherent difficulties in reading or setting the measuring means. PA1 1. Simplicity of design and manufacture leading to minimum possibility of absolute errors in construction. PA1 2. Avoidance of the possibility of backlash, or of accumulative, progressive or relative errors. PA1 3. Avoidance of error resulting from strain on machine parts. PA1 4. Minimization of errors due to machine misalignment or operator carelessness. PA1 5. Capability of being read continuously without change or adjustment and the capacity to measure in both the positive and negative directions. PA1 6. Availability of a full floating zero or datum point. PA1 7. Capability of measuring at machine movement speeds in excess of 200 inches per minute and over distances of up to 50 feet. PA1 8. Read-out capability in either the English or metric systems.
A. a cylinder or disc of known diameter is held against a flat surface so that relative motion of the cylinder support means and the surface causes the cylinder to rotate, or PA2 B. a cable or tape is wrapped about a cylinder in such a way that rotation of the cylinder is produced by frictional engagement between the cylinder axis and the capable or tape.
Either initially or with the passage of time, all of the above systems prove subject to one or more of the following deficiencies:
The many expedients adopted by various manufacturers to minimize these difficulties have inevitably tended to multiply costs without in any case providing a fully satisfactory solution to the underlying problem.
It is an object of the present invention to provide lost cost measuring apparatus which has the following advantages over prior devices:
The invention employs the concept that a flexible tape engaging a rotatable cylinder of known dimensions, can, upon relative motion between the tape and the cylinder surface, cause the resulting rotation of the cylinder to perform a measuring function. However, the invention provides means for overcoming the defects of prior tape driven systems which have militated against satisfactory performance of such systems in situations in which very high precision measurement is essential. In particular, arrangements are provided which entirely avoid reliance upon surface friction as a means for producing relative motion between engaging parts.
More specifically, the invention employs an extended tape (or similar continuous elongated flexible member) having a number of equally spaced, precisely formed holes, the spacing of which is exactly known as long as the tape is subjected to appropriate longitudinal tensile stress. Combined with the tape is a cylinder having its diameter related in a critical way both to the parameters of the measuring system which is to be employed and to the location of the pitch line of the tape as it is bent around the periphery of the cylinder. In operation, the tape is maintained in a partial wrap-around relationship with the cylinder. Precisely controlled relative motion between the tape and cylinder during rotation of the latter is achieved by provision on the cylinder of tape-engaging elements, the spacing of which (measured along a critical circle), is precisely that of the tape openings, and the configuration of which is appropriately related to that of such openings. The cylinder is connected through high quality gearing to a precisely calibrated read-out system, the reading of which is controlled by rotation of the cylinder as effected by relative motion between it and the tape.
In applying the combination as so far described to a structural system (e.g. a machine tool) having a movable part and a fixed part, attachment means are provided such that the axis of the cylinder is maintained transverse to the direction of motion of the movable part and the principal longitudinal extension of the tape is maintained parallel to that direction. Either the extremities of the tape are attached to the movable part of the structural system with the cylinder being borne by the fixed part, or the reverse condition is maintained. Under either of these conditions, it is found that the relative motion of the tape and cylinder which occurs when the movable part is shifted from one position to another will, through the resulting rotation of the cylinder, produce a read-out of the part movement with an accuracy (in the English system) of or better than one ten-thousandth of an inch. By operator switch-in of a translation gear chain between the cylinder and the read-out mechanism, read-out with similar accuraucy can immediately be obtained in the metric system.
Particulars of the invention, as well as its further objects and advantages, may best be understood by reference to the following detailed description, taken in connection with the accompanying drawings.